Re: [ferret_users] Plotting differences is not giving correct answer, time axis problem?

To: Glenn Carver <Glenn.Carver@xxxxxxxxxxxxxxxx>
Subject: Re: [ferret_users] Plotting differences is not giving correct answer, time axis problem?
From: Jaison Kurian <jaison@xxxxxxxxxxxxxxxxxx>
Date: Tue, 10 Jul 2007 13:42:59 +0530 (IST)
Cc: ferret_users@xxxxxxxx
In-reply-to: <p05111b09c2b8718eb287@[10.0.1.6]>
References: <p05111b09c2b8718eb287@[10.0.1.6]>
Sender: owner-ferret_users@xxxxxxxx

Hi Glenn,
           In one sentence : it will not be a problem with time axis, but
may be with "time region" settings. There are 3 points to noticed, which are
discussed below. The solution to your problem (i guess so...) will be 1, and
2-3 will give you some general stuff regarding LIST and @AVE.

 1. According to the header part of the LIST output, the entries in each
       Column are :

      1st : NO2_MM[D=ptom_t042_199703mm,T=31-MAR-1997 00:00]
      2nd : NO2[D=ptom_t042_1997030106,T=01-MAR-1997 03:00:31-MAR-1997 21:00@AV]
      3rd : NO2_MM[D=ptom_t042_199703mm]-NO2[D=ptom_t042_1997030106,T=@AVE]

    I guess that your monthly data have a time stamp for the end of the month
    (31-MAR-1997 00:00). Now, in 2nd column, you have a variable which is precisely
    averaged over "01-MAR-1997 03:00:31-MAR-1997 21:00". But the third column is
    NOT the direct difference between first 2. Here your first varible NO2_MM does 
    not have any time mentioned, but assume that either you have only one time point
    for this variable or you have fixed the time point somewhere in the script. Now
    the problem is with the second variable NO2, IT DOES NOT HAVE any specified time
    region, it averages over THE WHOLE time period as T=@AVE.

    In summary, if your NO2 variable spill over March-1997, or if you have not set 
    the time region to "01-MAR-1997 03:00:31-MAR-1997 21:00" explicitly, you will 
    not get the 3rd column as the difference between 1st and 2nd.

 2. The real reason for your problem may be 1 above. However, when you check the 
    accuracy of a calculations using list command, always use HIGH PRECISION for 
    your list command, as follows :

          yes? set list/precision=7
          yes? list var1, var2, var3

    Please note that, specifying precision along with the list command may not be
    effective, if more than one variable is being listed (see this recent posting
    http://www.pmel.noaa.gov/maillists/tmap/ferret_users/fu_2007/msg00374.html).

 3. The @AVE regridding is a grid-box weighted transformation, when you use it over
    coordinate axes like X,Y,Z and T (but not with the abstract axes I,J,K and L).
    Hence, if the region is not specified for exact grid boundaries, the resulting 
    averaged values may be (slightly) different from what you expected. For more
    information, please see,

    User Manual : Ch3 Sec2.4.6.  @AVE-average
    FAQ  
         http://www.ferret.noaa.gov/Ferret/FAQ/analysis/regridding_with_ave.html

    Mail archives (discussions starting with the following one)
         http://www.pmel.noaa.gov/maillists/tmap/ferret_users/fu_2006/msg00679.html
   
Please let me know, if you have any questions.

Regards,

Jaison


On Tue, 10 Jul 2007, Glenn Carver wrote:

> Dear Ferret fans,
> 
> I've been scratching my head over some results Ferret is giving me. I 
> am trying to plot the difference between two fields of NO2.   The 
> data from the fields is shown below. In the first column I have a 
> monthly mean NO2 field (variable no2_mm in the netcdf file). In the 
> second column I have a time average computed from 6hrly output, ie. 
> no2[t=@ave] where the data in the file runs from 1st March to 31st 
> March.
> 
> If I try to take the difference as shown in the 3rd column I get 
> wrong answers.  When I do the difference I get a warning from Ferret 
> that the time axes are not compatible (*** NOTE: Ambiguous 
> coordinates on T axis: NO2_MM[D=1]-NO2[D=2,T=@AVE]) ; presumably 
> because my monthly mean field has a date of 31/3 whereas ferret uses 
> a date in the middle of the month?
> 
> But I  can't figure out what Ferret is doing. It's obviously not 
> subtracting the numbers in the way I want to. My guess is it's 
> shifted the time axis somehow?   The differences around 87W for 
> example are clearly not calculated right.
> 
> Apart from the fact this is rather scary, how do I fix it?  Thanks.
> 
>      Glenn
> 
> --
> 
>               LATITUDE: 40.5N
>               Z (sigma_level): 31
>   Column  1: NO2_MM[D=ptom_t042_199703mm,T=31-MAR-1997 00:00] is 
> Nitrogen Dioxide monthly mean (ppbv)
>   Column  2: NO2[D=ptom_t042_1997030106,T=01-MAR-1997 
> 03:00:31-MAR-1997 21:00@AV] is Nitrogen Dioxide (ppbv)
>   Column  3: EX#3 is 
> NO2_MM[D=ptom_t042_199703mm]-NO2[D=ptom_t042_1997030106,T=@AVE]
>               NO2_MM    NO2   EX#3
> 180E   /  65:   0.02   0.02 -0.003
> 177.2W /  66:   0.02   0.02 -0.002
> 174.4W /  67:   0.02   0.02  0.001
> 171.6W /  68:   0.02   0.02  0.003
> 168.8W /  69:   0.02   0.02  0.007
> 165.9W /  70:   0.02   0.02  0.006
> 163.1W /  71:   0.02   0.02  0.003
> 160.3W /  72:   0.02   0.02  0.000
> 157.5W /  73:   0.02   0.02 -0.005
> 154.7W /  74:   0.02   0.02 -0.008
> 151.9W /  75:   0.02   0.02 -0.007
> 149.1W /  76:   0.02   0.02 -0.005
> 146.2W /  77:   0.02   0.02 -0.005
> 143.4W /  78:   0.02   0.02 -0.006
> 140.6W /  79:   0.02   0.02 -0.009
> 137.8W /  80:   0.04   0.04  0.003
> 135W   /  81:   0.07   0.06  0.027
> 132.2W /  82:   0.07   0.07  0.038
> 129.4W /  83:   0.04   0.04  0.017
> 126.6W /  84:   0.04   0.04  0.019
> 123.8W /  85:   0.32   0.31  0.144
> 120.9W /  86:   0.67   0.67 -0.216
> 118.1W /  87:   0.40   0.40 -0.206
> 115.3W /  88:   0.26   0.25 -0.220
> 112.5W /  89:   1.88   1.87 -0.096
> 109.7W /  90:   1.55   1.50  0.221
> 106.9W /  91:   1.65   1.62  0.250
> 104.1W /  92:   1.96   1.94  0.094
> 101.2W /  93:   1.25   1.26  0.091
> 98.4W  /  94:   1.58   1.64  0.330
> 95.6W  /  95:   3.38   3.48  1.099
> 92.8W  /  96:   2.39   2.41  0.960
> 90W    /  97:   4.17   4.23  1.620
> 87.2W  /  98:   6.94   7.22  2.057
> 84.4W  /  99:   5.84   6.09  2.405
> 81.6W  / 100:   7.34   7.85  0.619
> 78.8W  / 101:   5.93   6.23  0.376
> 75.9W  / 102:  12.74  13.41 -1.871
> 73.1W  / 103:   6.06   6.32 -1.817
> 70.3W  / 104:   1.52   1.56 -0.706
> 67.5W  / 105:   0.80   0.81 -0.422
> 64.7W  / 106:   0.36   0.37  0.109
> 61.9W  / 107:   0.24   0.24  0.079
> 59.1W  / 108:   0.17   0.17  0.094
> 56.2W  / 109:   0.14   0.14  0.088
> 53.4W  / 110:   0.10   0.10  0.073
> 50.6W  / 111:   0.08   0.08  0.054
> 47.8W  / 112:   0.08   0.08  0.036
> 45W    / 113:   0.08   0.08  0.006
> 42.2W  / 114:   0.07   0.07 -0.010
> 39.4W  / 115:   0.07   0.07 -0.009
> 36.6W  / 116:   0.10   0.10  0.006
> 33.8W  / 117:   0.22   0.22  0.023
> 30.9W  / 118:   0.16   0.16 -0.039
> 28.1W  / 119:   0.06   0.05  0.012
> 25.3W  / 120:   0.09   0.09  0.009
> 22.5W  / 121:   0.22   0.21 -0.023
> 19.7W  / 122:   0.25   0.24 -0.038
> 16.9W  / 123:   0.26   0.26 -0.093
> 14.1W  / 124:   0.40   0.40  0.026
> 11.2W  / 125:   1.44   1.43 -0.135
> 8.4W   / 126:   0.98   0.99  0.077
> 5.6W   / 127:   1.46   1.54  0.214
> 2.8W   / 128:   1.57   1.63  0.168
> 0E     / 129:   0.93   0.93  0.103
> 
> 

-- 
This message has been scanned for viruses and
dangerous content by MailScanner, and is
believed to be clean.

References:
- [ferret_users] Plotting differences is not giving correct answer, time axis problem?
  - From: Glenn Carver

Previous by thread: [ferret_users] Plotting differences is not giving correct answer, time axis problem?
Next by thread: [ferret_users] vectors on polar stereographic map projection

[Thread Prev][Thread Next][Index]

Dept of Commerce / NOAA / OAR / PMEL / TMAP